1. Field of the Invention
The present invention relates to a method and control system for adjusting air/fuel ratio in cylinders of an internal combustion engine in anticipation of an impending engine event that will change exhaust emissions to precondition a catalyst in a manner to reduce emissions when the event subsequently occurs.
2. Description of Related Art
To reduce emissions, such as NOx, HC, and CO, modern automotive vehicles typically include an emission control system coupled to the engine of the vehicle. For example, many vehicles are equipped with a three-way catalytic converter, which includes a catalyst material capable of storing oxygen and NOx (oxidants) during periods when the engine is operated with a lean air/fuel ratio and of releasing stored oxygen and NOx for reaction with HC and CO (reductants) produced by the engine during periods when the engine is operated with a rich air/fuel ratio. In this way, the emission of both oxidants (NOx) and reductants (HC and CO) into the atmosphere is reduced.
Most emission control systems are employed in connection with an engine air/fuel ratio control strategy that monitors and adjusts the air/fuel ratio provided to the engine cylinders in order to increase the emission reduction capability of the catalyst. The air/fuel strategy typically attempts to maintain the engine at stoichiometry (or other preselected desired air/fuel ratio) and relies on air/fuel feedback from one or more exhaust gas oxygen sensors. For example, it is known to position a pre-catalyst exhaust gas oxygen sensor upstream of the catalytic converter and a post-catalyst exhaust gas oxygen sensor downstream thereof. The output signals from the pre-catalyst and post-catalyst exhaust gas oxygen sensors are each compared to a respective set point reference value to calculate a pre-catalyst error value and a post-catalyst error value. The error values are generally indicative of whether the air/fuel ratio at the point of the respective exhaust gas oxygen sensor is rich or lean relative to stoichiometry. An electronic engine controller adjusts an amount of fuel provided to the engine cylinders, and thus the air/fuel ratio therein, based at least in part on the pre-catalyst and post-catalyst error values.
However, conventional air/fuel ratio control strategies do not attempt to anticipate a future engine operating event that may lead to unwanted emissions. For example, conventional air/fuel ratio control strategies do not attempt to anticipate a so-called impending engine tip-in operating event that increases engine loads (e.g. vehicle acceleration from stop or near stop) where elevated NOx concentrations are present in the engine exhaust gas as a result of increased in-cylinder gas temperature and pressure. If the emission control system waits to actually observe and then counter the tip-in operating event by providing a rich air/fuel ratio, then unwanted emissions of NOx can occur as a result of oxidant saturation of the catalyst from a previous engine operating event, such as a previous tip-in event, elevated speed cruising event, and lean deceleration, thereby degrading emission control.